1. Field of the Invention
The present invention relates to a halftone recording method and apparatus for performing recording on a recording sheet at a plurality of gradation levels while moving the recording sheet.
2. Description of the Prior Art
Among many recording apparatuses for performing halftone recording, a halftone recording apparatus using a thermal recording head such as a thermal head is widely applied to various recording apparatuses such as a printer, a copier, and a facsimile machine because of its advantages of simple mechanism, high reliability, superior maintainability, etc. The halftone recording apparatus using a thermal recording head performs recording of n gradation levels (n: integer of 2 or larger) on a recording sheet by thermally transferring ink of an ink sheet of a melting type, a sublimation type, or some other type.
As described above, to perform halftone recording, i.e., recording of n gradation levels (n is 64, for instance), a thermal transfer recording method is employed which uses a melting or sublimation type ink sheet. In this thermal transfer recording method, desired recording is performed by transferring, to a prescribed recording sheet, pigment or dye ink of a quantity that corresponds to a heat quantity generated by electric energy that is applied to each of heating resistors constituting a thermal recording head. The heat quantity of each heating element is controlled by the number or duration of pulses applied to it.
The above conventional recording method is described, for instance, in Japanese Unexamined Patent Publication No. Sho. 60-9271. FIG. 35 is a waveform diagram showing energizing pulses SB applied to each of the heating resistors that constitute the thermal head in the conventional halftone recording method under consideration. In FIG. 35, t.sub.W denotes a width of the pulse SB; t.sub.P, a repetition period of the pulse SB; and N (3 in this example), the number of pulses SB. The number N of pulses SB is preset for the density of each gradation level.
By applying, to each heating resistor, energizing pulses SB of a number corresponding to a desired gradation level in the above manner, ink of a quantity corresponding to energy, i.e., the number of pulses is transferred, to thereby effect halftone recording of each density.
Usually, recording of one line is performed by applying, en bloc or in a divided manner, energizing pulses to the respective heating resistors that are aligned in the thermal head. Two-dimensional recording is effected by sequentially performing recording of respective lines while feeding a recording sheet at a constant speed in the auxiliary scanning direction.
However, for the following reason, it is difficult for the above halftone recording method to realize recording of high image quality. Since the recording density of each gradation level is mainly determined by the temperature of the heating resistors of the thermal head, a temperature variation due to a variation among the heating resistors, a variation of the environment temperature, heat storage, and other factors greatly affect the recording density. To solve this problem, many correcting measures have been proposed.
To correct for a variation of the environment temperature, there is known a correcting method in which a temperature variation is detected by, for instance, a thermistor attached to the thermal head and the width or number of pulses that is given for each gradation level is controlled. Thus, a variation of the recording density for the same gradation level is suppressed.
To correct for density unevenness due to a variation in resistance among the heating resistors, for instance, a paper entitled "Development of High-quality Video Copying" (Proceedings of the '86 General National Conference of the Institute of Electronics, Information and Communication Engineers of Japan, No. 1,276) states that the following measure is effective.
FIG. 36 is a block diagram showing an example of such a resistance correcting measure. In FIG. 36, a counter 101 counts clock pulses. An EPROM 102 receives a count value of the counter 101 as an address and outputs corresponding data (a correction coefficient number for a heating resistor). An EPROM 103 receives, as addresses, the output data of the EPROM 102 and 6-bit signals of cyan (C), magenta (M) and yellow (Y), and outputs corresponding data (for instance, resistance-corrected 6-bit C, M and Y signals).
A description will be made of the operation of the above circuit. Resistance values of the respective heating resistors are measured in advance, and the heating elements are grouped according to the measured resistance values. The EPROM 102 stores correction coefficients for the respective groups. Based on a signal sent from the counter 101 and indicating a heating resistor, the number table of the EPROM 102 is retrieved and a correction coefficient set number for the heating resistor is selected. Based on the selected correction coefficient set number, the EPROM 103 changes the magnitudes of the C, M and Y signals. That is, in response to an input signal having a gradation level somewhere between the 0th and 63rd levels, the EPROM 103 generates a signal having a gradation level also somewhere between the 0th and 63rd levels in accordance with the information on a variation of a heating resistor that is supplied from the EPROM 102. For example, when the ideal gradation level corresponding to the 1,000th heating resistor is the 38th level, the signal is corrected to the 40th or 35th level.
However, to realize recording of high image quality, this correction method requires a correction table that contains an enormous amount of data, resulting in a very expensive, large-sized apparatus. For example, where the number of bits of both of the gradation data signals and the correction coefficients of the EPROM 102 is increased to 8 to realize recording of high image quality, the EPROM 103 should have a capacity of 512 Kbits.
On the other hand, recently, a printer is marketed which can perform both a sublimation type operation capable of providing high image quality and the melting type operation capable of providing high-speed recording. In this type of printer, a melting type ink sheet or a sublimation type ink sheet is mounted on a conventional halftone recording apparatus.
However, to realize halftone recording either by using both of the melting type and sublimation type operations or by using one of those, problems should be solved which relate to the size of the heating resistors of the thermal head and the head driving method.
In the sublimation type recording, since the density is modulated within one pixel, the temperature distribution of the heating elements of the thermal head is desired to be uniform in each pixel. Each heating resistor has a long and narrow shape. Ideally, the width in the main scanning direction and the length in the auxiliary scanning direction of the heating resistor should be approximately the same. However, in this case, horizontal streaks likely occur in low-gradation recording, thus deteriorating image quality. This phenomenon in low-gradation recording occurs such that each heating resistor has a temperature distribution that is longer in the horizontal direction and a gap is thereby formed between the corresponding pixel of the next line. For example, with resolution of 300 DPI, the width in the main scanning direction is about 83 .mu.m and the length in the auxiliary scanning direction is about 200 .mu.m. As described above, to suppress the occurrence of horizontal streaks, each heating resistor of the thermal head are made long and narrow while its aspect ratio is increased as a tradeoff. (The aspect ratio is a ratio between widths of recorded horizontal and vertical lines. In the above example, a horizontal width of about 83 .mu.m and a vertical width of about 200 .mu.m produces an aspect ratio of 2.4.)
On the other hand, since the melting type recording employs an area modulation method in which the transfer area is modulated within one pixel, the temperature distribution of the heating resistors is desired to assume concentric circles. To provide better sharpness and stability of recorded pixels, it is preferred that there exist a certain temperature difference. From this point of view, the shape of each heating resistor of the thermal head should be close to a square. However, in the melting type recording, each pixel is influenced by adjacent pixels more likely than in the sublimation type recording. Even if heat control is so performed as to refer to adjacent pixels, it is difficult to provide stable gradational expressions.
With the above described features, where the heating resistors that are usually used for the sublimation type recording are used for the melting type recording, a low-gradation portion may become too thin or have too low a density and overtransfer may occur in a high-gradation portion. Further, the density increases too steeply with the energy applied. Thus, it is difficult to perform proper gradational recording. Even where the heating resistors are used for the original sublimation type recording, an excessively large aspect ratio will prevent sufficient quality for characters and a line image. Where the heating resistors having a shape suitable for the melting type recording are used for the sublimation type recording, horizontal streaks will deteriorate the image quality.
The problems of the above-described conventional halftone recording method and apparatus are summarized below.
First, as described below, in the conventional halftone recording method and apparatus, particularly in the sublimation type recording, horizontal streaks may occur and sufficient quality will not be obtained for characters and a line image.
Second, driving with the thermal head using either the long and narrow heating resistors or the square heating resistors cannot accommodate both of the melting type and sublimation type recording methods at the same time, that is, causes insufficient image quality in one of the two methods.
Third, burning in high-gradation recording deteriorates image quality. A plurality of energizing pulses are applied to realize n gradation levels. In high-speed recording, the surface of a recording sheet may be scorched when the temperature of the thermal head becomes too high.
Fourth, in the conventional halftone recording apparatus, a less expensive recording head for binary use is employed, and n-gradation recording is realized by n-1 times of data transfer and n-1 times of energization. This driving method has a problem that where n is increased to, for instance, 256 or 512 to attain recording of high image quality, the transfer time becomes very long to disable high-speed recording. Further, to realize recording of high image quality, each correction means should be very precise. This will increase the volume of tables, making the apparatus large and expensive.